partly revert e3bb221: properly save drush output
the commit's assumption was incorrect: HOSTING_DRUSH_OUTPUT is the
actual return value of a single drush_backend_invoke() call, not a
list of such output or the actual drush output (that's the 'output'
field of the return value).
so we need to set HOSTING_DRUSH_OUTPUT to the output of the actual
task, and that's it. we can get the log output of the provision-save
commands by using the integrate parameter, which this patch does.
properly import db_name after site import
45a80e5bb from #2470297 introduced a regression where the db_name was
lost on imports. that is because the db_server module doesn't implement properly the context_import hook. this was previously done by hand in hosting_site_post_hosting_import_task() but that hardly seemed like the right place of doing this - this belongs to the DB server.